Enhanced selective nitrate-to-nitrogen electrocatalytic reduction by CNTs doped Ni foam/Cu electrode coupled with Cl−

In this work, a Ni foam/CNTs/Cu composite electrode was prepared via the impregnation and electrodeposition methods. The electrode enhanced the total nitrogen removal and N2 selectivity, when served as a cathode for NO3−-N reduction. SEM, EDS, XRD, and XPS characterization revealed that both CNTs and metallic copper were introduced onto the surface of the Ni foam substrate successfully. Besides, the LSV, CV, EIS and Cdl results indicated more electrochemical active sites were in the Ni foam/CNTs/Cu electrodes than in Ni foam/Cu electrodes. Under optimal preparation conditions, the Ni foam/CNTs/Cu electrode showed N2 selectivity of 68.89 % and nitrate removal rate of 96.75 % in 100 mg/L nitrate solution with Na2SO4 as electrolyte. The selectivity of N2 could be significantly enhanced by adding Cl− as electrolyte, and the nitrogen selectivity increased with the increase of Cl− concentration. At 1.25 g/L NaCl, the Ni foam/CNTs/Cu electrode could achieve >95 % nitrate and total nitrogen removal and close to 100 % N2 selectivity. The N2 selectivity of the Ni foam/CNTs/Cu electrode was 2.534 and 1.8 times of the Ni foam/Cu electrode when 0.5 g/L Na2SO4 and 0.5 g/L NaCl were used as electrolytes, respectively. Consequently, to achieve the same nitrate reduction effect, the Ni foam/CNTs/Cu electrode could reduce the amount of NaCl compared with Ni foam/Cu electrode. The electrodes were tested in 6 cycles and proved to be stable. At last, a potential mechanism of nitrate‑nitrogen removal by the electrocatalytic process was suggested.